Actuators are devices that generate displacement or force for various applications. These can take on many forms such as motors, air cylinders, hydraulic cylinders, and electromagnetic solenoids to name a few. These actuators are utilized for many different applications and have been in use for decades.
A particular class of polymeric based actuators that have been developed are polymer actuators based on elastic epoxy hydrogel polymers. Such activators work by creating ionic imbalances near electrodes using positive or negative electrical charges at an electrode in contact with or in close proximity to the polymer and an ionic species of the electrolyte. By increasing the density or the number of the amine groups, such as secondary and primary amines, an increase in swelling pressure is attained in the hydrogel. This also has an effect on the swelling and deswelling times.
One way of achieving this is by incorporating Polyamidoamine (PAMAM) polyether dendrimers, poly(propylene imine (PPI)-dendrimers, amino functionalized dendrimers, or combinations thereof, as part of the polymer structure. Dendrimers are highly branched and offer superior numbers of polymer linkage points. They also are commercially available with primary amino surface groups and core amino groups. This makes engineering of the hydrogel possible so that specific performance parameters such as the pressure the gel can produce is determined by formula ratios of materials or by controlling the organization, size and number of branches in polymer structure itself. Hydrogel density and porosity is controlled by the amount of amine functionality and molecular weight of the polyether amines. Hydrogel density and porosity is also controlled by amount of polyethylene glycol diglycidyl ether and/or by the ratio of H2O or solvent used to polymerize the materials.
A preferred ether for this gel is polyethyleneglycol-dodecylether (polyEGDE), but other ethers also can be used such as cyclohexanedimethanol diglycidyl ether. These ethers produce a very clear and strong hydrogel that reacts hydrophobically to high pH aqueous solutions and swells when exposed to low pH or acidic solutions. Hydrogel density and porosity also can be controlled by adding an amount of oxidizer to the polymer during polymerization. Whether in solution or dry these oxidizers can be further activated chemically electrically or by photons during polymerization to achieve desired properties.
Ionic hydrogel swelling kinetics are achieved by a difference in pH, ions, cations or protons between a solution outside of the hydrogel and a solution inside of the hydrogel or the polymer composition of the hydrogel. These performance characteristics can be controlled several ways. For example, adding acid to the polymer during polymerization creates a hydrogel that has a higher pH swelling property. Hydrogel swelling kinetics also can be controlled by adding salts or alkali solutions to the polymer during polymerization. This is accomplished by chemical, electrical, electrochemical, photo or photochemical excitation of the epoxy polymer or solution that it is hydrated with.
It is possible to create an electro activated polymer (EAP) by hydrating the epoxy hydrogel in an electrolyte, inserting an electrode into the gel, and spacing a second electrode a short distance from the hydrogel and running low amounts of current through the electrodes. For example, epoxy hydrogel swelling may be increased in the region of a platinum electrode using saline as an electrolyte fluid. When the polarity is reversed, the hydrogel will deswell or contract. Control of hydrophobic and hydrophilic properties also can be achieved by these methods.
A challenge with these polymer actuators is to generate forces and strain rates of actuation that are sufficient for pumping fluids.
Another challenge with such polymeric actuators is the generation of gas during operation. Polymeric actuators utilizing electrodes and aqueous electrolytes are particularly advantageous for high efficiency actuation but may hydrolyze out oxygen and hydrogen.
Gases produced in a polymer actuator are undesirable for the effects they may have on charge transfer and the mechanical system. By producing an insulating pocket, a gas bubble will impede charge transfer through the system. This effectively lowers the surface area of the electrode and/or the polymer which reduces the rate of expansion of the polymer. To make matters worse, the gas pocket is variable and unpredictable in size and precise composition, reducing accuracy of any device utilizing a polymeric actuator.
Also, gas expansion may produce an apparent expansion of an actuator that later relaxes due to compression or absorption of the gas. Gas production thus reduces repeatability, predictability, and accuracy of the actuator.
Compounding these consistency problems is the inconsistent adhesion of actuator polymer to a non-reactive substrate. Hydrogels are prone to delamination and typically are not stable in aqueous environments, hindering, for example, thin film characterization and use in aqueous media. Adhesion may require solutions that increase the number of chemicals and process steps leading to increased complexity, and may use chemicals that are not environmentally benign.
There is therefore a need for an actuator that can produce high rates of actuation without the shortcomings of hydrolysis or other gas generation.
There is further a need for a hydrogel actuator that is conveniently adhered to a hydrophobic surface.
Conventional actuators do not meet the full need for such an application. Furthermore state of the art polymer actuators do not isolate and create distinct ionic or pH boundaries of the electrolyte at the anode or cathode side of the electrodes.
Typical electrode materials for polymer actuators are metals, carbons and some conductive polymers. The type of electrode is somewhat dependent on the type of actuator and electrolytic chemistry needed to create the desired response. One drawback to using most metals is that they typically are not stable for both the oxidation and redox reaction at each electrode. Nobel metals such as gold and platinum can be used due to their stability but are very expensive and not practical for industrial type use. Other materials such as carbon and graphite will work but have difficult working mechanical properties making them not practical, and conductive polymers are very limited in their chemical compatibility to other polymers. Another drawback all electrodes have is electrochemical gas generation at the electrode surface interface with the electrolyte. The generation of gas, both in type and volume, is dependent on the polarity of the electrode and the amount of electrical current applied to the electrodes in an actuator or actuator assembly and the electrolyte. Gas creates a significant problem of compressible pressure in closed or sealed actuator systems, and can actually outpace the polymer actuation or volume change thereby creating unreliable results in actuation cycles.